A semiconductor device (semiconductor integrated circuit device) is produced by repeatedly using photolithography in which exposure light is irradiated onto a mask of an original plate, on which a circuit pattern is drawn, so as to transfer the circuit pattern on a main plane of a semiconductor substrate (hereinafter, referred to as wafer) through a reduced projection optical system.
However, in recent years, in response to demand for microfabrication of the semiconductor device, development of EUV lithography (hereinafter, referred to as EUVL) using EUV light whose wavelength is shorter than that of the light used for the exposure of the photolithography has been advanced. By using this EUVL, resolution can be improved, and a further-microfabricated circuit pattern can be transferred.
In a wavelength range of the EUV light (whose center wavelength is, for example, 13.5 nm), a transparency mask cannot be used because of light absorption of its material. Therefore, a multi-layered reflection substrate which utilizes reflection by a multi-layered film made of molybdenum (Mo), silicon (Si), and others, is used as an EUVL mask blank (hereinafter, referred to as mask blank). An EUVL mask is configured by forming an absorber pattern on a plane of this mask blank (see in, for example, “Introduction to Photomask Technology”, Kogyo Chousakai Publishing Co., Ltd., written by Isao TANABE, Yohichi TAKEHANA, and Morihisa HOUGA, published on December 2006, pp. 266 to 268 (Non-Patent Document 1)).
Also, since a transparent lens cannot be used, reflection-type exposure optical system (reflection-type imaging optical system, EUV optical system) made of only a reflection plane of a multi-layered film obtained by alternately stacking Molybdenum (Mo) and silicon (Si) is used for the reduced projection optical system as described in, for example, Japanese Patent Application Laid-Open Publication No. 2007-158828 (Patent Document 1). The light from a light source is homogenized through a reflection-type illumination optical system, and is irradiated to the EUVL mask. The light irradiated to the EUVL mask reflects on the EUVL mask, and reaches the wafer through a reflection-type projection optical system, so that the absorber pattern of the EUVL mask is projected on the main plane of the wafer.
In the EUVL, even when slight height abnormality of about several nm occurs in the plane of the mask blank, the height abnormality results in large change in phase of the EUV reflection light, and results in defects such as dimensional change or failure of resolution in the transfer of the absorber pattern onto the main plane of the wafer. Such a defect which results in the phase change is called phase defect. Accordingly, it is required to detect the phase defect at a stage of a mask blank obtained prior to coating of the absorber pattern.
As a general method of inspecting the mask blank, there are a method of detecting a foreign material and a method of detecting a bright-field image (microscope image) from diffused reflection light caused by irradiating laser light to the mask blank. However, the influence of the phase defect also depends on an internal structure of the multi-layered film, and therefore, it is considered that an inspection method at wavelength (the same wavelength) of detecting the defect with using detection light whose wavelength is the same as that of the EUV light used for the exposure is suitable. As one example of this method, for example, Japanese Patent Application Laid-Open Publication No. 2003-114200 (Patent Document 2) discloses a method with using a dark-field image inspection image. Also, for example, Japanese Patent Application Laid-Open Publication No. 2007-219130 (Patent Document 3) discloses an inspection method of differentiating concavity and convexity of a plane of the phase defect. Further, for example, Japanese Patent Application Laid-Open Publication (Translation of PCT Application) No. 2002-532738 (Patent Document 4) discloses a technique of improving a projection image in pattern transfer by an exposure device by adjusting a contour of the absorber pattern if the absorber pattern is formed in a state that the phase defect already exists.